Mixed modulation in pulse intercommunication systems



Sept- 9, 1952 E. M. DELORAINE ET AL 2,610,254

MIXED MODULATION IN PULSE INTERCOMMUNICATION SYSTEMS 7 Sheets-Sheet l Filed Dec.

HHIHIH Gn mom Qwto N ok m In venters 'ED/WONU M. DfOPlq//Vf mfc /Mmfy #ffl/fs Sept. 9, 1952 E. M. DELORAINE ET AL 2,610,254

MIXED MODULATIGN 1N PULSE INTERCOMMUNICATION SYSTEMS Filed Dec. 4, 1948 7 Sheets-Sheet 2 A Home Sept. 9, 1952 E. M. DELORAINE ET A1. 2,610,254

MIXED MODULATIGN 1N PULSE INTERCOMMUNICATION SYSTEMS F/GAB.

A torney Sept 9, 1952 E. M. DELQRAINE ET Ax. 2,610,254

MIXED MODULATION IN PULSE INTERCOMMUNICATION SYSTEMS Filed Deo. 4, 1948 7 Sheets-Sheet 4 HIN] D Hllllslvl what 2mb @bw IIHIHH Aftorney Sept. 9, 1952 E. M. DELORAINE ET AL 2,610,254

MIXED MODULATIGN IN PULSE INTERCOMMUNICATION SYSTEMS Filed Dec. 4, 1948 7 sheds-sheet 5 n ventor 7 Sheets-Sheet 6 E. M. DELORAINE ET AL In venters HH/PLEY REEL/f5 y ttorney M A m M. M. 0 N m m A EC By @i774 MIXED MODULATION IN PULSE INTERCOMMUNICATION SYSTEMS Sept. 9, 1952 Filed Dec.

Sept. 9, 1952 E. M. DELORAINE ET A1. 2,610,254

MIXED MODULATION IN PULSE INTERCOMMUNICATION SYSTEMS Filed Dec. 4, 1948 7 Sheets-Sheet 7 Attorney Patented Sept. 9, 1952 AUNITED STATES PATENT FFICIE.v

MIXED MODULATION IN PULSE INTER- ,CGMMUNICATION SYSTEMS .Application December 4, 1948, Serial No; 63,592 In Great Britain December.7, 1347 16C1aims. 1

, This :invention relates toV telephone exchange systems wherein intelligence is transmitted in the form of electric pulses modulated by the, intelligence, and concerns the transmission problems encountered in such systems.

vSystems of this `type have been proposed wherebyanumberof different channels are provided in theiorm of different time positionsin a recurring cycle of time positions which is sometimes referred toas time division multiplex.

In av systemcateringforA a large number of substations the number of time positions required, in-central partsoflthesystem, to accommodate'the necessary Vnumber of channels may well be large and vinvolve the use ofclosely spaced pulses.

'I'he term telephone exchange system used in this speciiication means either a single main exchange and its subscriber network or a network of `relatively closely-spaced exchanges between which direct current dialling vand supervision have normally been used, e. g. the network of exchanges in a built-up area.

According to one of its features,` the invention `consists of a telephone exchange system'whereinl communication channels take the form of time positions in a recurring cycle of time positions, characterized in this that two ormore such recurring cycles of time positions form individual links in the said systemfand that two or more time cycles, forming links in the system are conlcentrated together by re-timing and combination ofthe individual time cycles `thereof in a single `cycle of time positions forming a link in the system the time positions of suchsingle time cycle being of shorter duration and higher repetition frequency than is the case with theV said two or more time cycles so concentrated.

According Vto another of its features the invention consists of a telephone exchange system wherein communication channels take the form of time positions in a recurringcycle of time positions, wherein intelligence is conveyed on such channels by electric pulses one or more characteristics of which is or are modulated in conformity with the said intelligence characterised in this that two or more such recurring cycles of time positions form individual links in the system and Athat the way of modulation ofthe said pulses andthe intelligence-modulation `relation are different-as between diiierent links of the 'said system `and that conversion `from one way of modulation to another andirom one intelligence-modulation relation `to another is efected 'asbetween diiierentlinks of the `system whereby that way `of modulation andinte1ligence-modulation relation is used in eachjlnk ,ofthe system which is best adapted to the transmission conditions in that link.

According to another of its features the invention consists of` a telephone exchange system wherein communication channels take the form of time positions in-a recurring cycle of time positions, wherein 4intelligence is conveyed on such channels by electric pulses `one or more characteristics of which is or are modulated-,in

conformity with the` said intelligence character- .ised in this that two or more such recurring cycles of time positions form individual links in ,the system fone or some of such links'` being more prone to cross-talk than the others and that the way of modulation and the intelligence modulation relationare .different as between `different `links ,of the system and that in a link or `.links mostlproneto cross-talk, away ofmodulation and intelligence-modulation relation `used `which gives a better performancewth respect to the avoidance of cross-talk than that of other ways of modulation and intelligence-modulation `relations 4used in other parts of `the system.

According to another of its featurestheinvention consists of a telephone-exchange system wherein communication channels take the form of time positions in a recurring cycle of time posi.- tions, whereinfintelligence is conveyed on such channels' by `ielectricqrpulses one or more characteristics of whichis or are modulated inconformity` with xthe` said intelligence characterised in this that two or moresuch'recurring cycles of timepositions form individuallinks in the system and that theway `of-mo'dulation of the said pulses `and the intelligence modulation relationv are different as between different links of the system, at least one of such diiierent ways of modulation and intelligence relation being of the type hereinafter described as pulse code modulation A, the use of which is coniined tcV links of the system which accommodate a larger number of `time positionsin a given time period than do other links of the system.

According to another ofvits features `then-invention consists of a telephone exchange system wherein communication channels take the'iorm of time positions in a recurring cycle of time position, wherein intelligence is conveyed on such channels Vby electric pulses one or morewcharacteristics of which is or are modulatedin conformity with the `said intelligence characterised in this that two or more such recurringcycles pf time positions form individuallinks initheisystexn Y and that the way of modulation of the said pulses and the intelligence-modulation relation are dif- Y ferent as between different links of the system,

the system which accommodate a larger number of time positions in a given time period than do other links of the system.

The expressions Way of modulation and intelligence-modulation relation used in the ve preceding paragraphs have the meanings hereinafter assigned to them in thisV specification.

The invention will now be described with reference to the accompanying drawing'sin which:

Fig. 1 is a schematic diagram of the transmitting portions of an embodiment according to the invention,

l Fig-2 is a series of graphs showing waveforms occurring at various parts of the equipment shown in Fig. 1,

Fig. 3 is a circuit diagram of part of the equipment shown schematically in Fig. 1.

Fig. 4 is a schematic diagram of the receiving portions of the said embodiment.

Fig. 5 is a circuit diagram of a part of the equipment shown schematically in Fig. 4.

Fig. 6 is a schematic diagram showing an alternative form of a part of the equipment shown schematically in Fig. 1.

Fig.7 is a series of graphs showing Vwaveforms occurring at various parts of the equipment shown in Fig. 4.

VA system such as that described of which some parts comprise individual groups of time division multiplex channels having a relatively small number of channels to accommodate and in which these time division multiplex groups are concentrated into a smaller number of time division multiplex super groups in central parts of the system, involves the transmission in a common .super-group transmission path of a larger number of channels than need be accommodated in one of the constituent groupsV and consequentlythe individual pulses must be of short duration and more closely spaced.

As pulses on a time division multiplex carry intelligence in the form of modulation of one or more characteristics of the pulses, and as a uniform standard of transmission fidelity is required throughout the system it would appear to be desirable to choose the form of modulation giving the required fidelity and to use it uniformly throughout the system wherever the intelligence is transmitted in pulse form.

In a system of the type described however the transmission problems are not uniform throughout the system and it is proposed to take the unusual step of using different types of modulation in different parts of the system choosing foreach link in the system the type of modulation best suited to the transmission characteristics of that link. It is found that the eXtra complication is amply justified by the resulting improvements in transmission performance.

The different transmission conditions yarise largely from the diiferent time division multiplex cycles used in the system. Some forms of modulation are preferable in some types of multiplex and other forms of modulation in other parts of the system, it is proposed to use different forms of modulation in different parts ofa system of the type described so as to have in each part of the system the most advantageous form 4 of modulation for the type of time division multiplex used in that part of the system. The advantage may of course be functional or economic or a combination of the two.

The Well known pulse` amplitude modulation, pulse width modulation Yand pulse time (or position) modulation'are simple and economical in apparatus but in general require a higher fre- Y quency band-width capacity in the transmission path than certain of the types of modulation which are referred to below.

A type of modulation has been proposed in U. S. Patent No. 2,272,070, issued February 3, 1942, where a number of fixed levels of signal amplitude are determined, the instantaneous value of a signal being defined as that one of these levels Vto which it is nearest at the instant in question. Each level may be related to a definite signal, characteristic of it, such as the ve unit code signal commonly used to transmit a teleprinter character, and the Yfixed level nearest t0 the instantaneous amplitude of the signal may then be transmitted by a pulse or a train of pulses of a type corresponding to that xed level according to the chosen code.

These code signals are practically immune fromV interference by other signals on other channel pulses and it has been found that speech may be dened with allowably small distortion by defining its instantaneous amplitudes in terms of 32 fixed levels which can be reproduced by a live-unitV code such as the teleprinter code.

It is possible of course to provide that the nearest fixed level above, or the nearest fixed level below, the instantaneous level of the intelligence waveform shall be used rather than the nearest level.

This type of modulation is hereinafter referred to as pulse code modulation A.

Somewhat similar types of modulation have been proposed in co-pending applications, Ser. No. 778,662 and Serial No. 778,663, led October 8, 1947. The waveform of an intelligence signal is dened by a positive pulse, a negative pulse or no pulse at all, the pulses occurring at fixed times and having xed amplitudes. The intelligence Waveform is observed at regularly spaced intervals coinciding With the pulse times and a positive pulse records a change in amplitude in the positive going direction between that pulse time and the preceding pulse time, in excess of a predetermined amount, the absence of a pulse records during a similar period no change in amplitude or a change less than the predetermined amount and a negative pulse, records a change in amplitude in the negative going direction, during a similar period, in excess of a predetermined amount.

The original intelligence waveform is reproduced by integrating devices which add the effects of successive pulses and the waveform is reproduced more or less accurately according to the closeness of the spacing of the coding pulses and the level of minimum change below which the pulse is absent.

The co-pending applications, Ser. No. 778,662 and Ser. No. 778,663 also described a variant of this system in which only two alternative indications are given which may be represented by a positive pulse and a negative pulse respectively.

Once started, on the transmission of a pulse of one polarity, pulses of that polarity will continue to be transmitted until their polarity is reversed in the following manner. The pulse output is integrated to provide a stepped Waveempates.

form whichy ist similar: to thatiati the. receivingi; end.;y This :.is, ay:.roughlrepresentatiomofr;the .sigle c nalf` wave form. A `comparator compares:y thisJ stepped waveforrrr` `witlrthat of: theincoming; intelligence and when successive pulses in one:`

direction have built up, atvoltage` at the-.output of thei integration Which-` is:A greater than.; ther intelligence 1 Waveforml voltage, the-.polarity ofv The process the;` outgoing pulsestisr reversed.. proceedsinthe other direction until theaintegrated waveform. againovertakes the intelligence: Waveform causing another reversal ofpulse.po.`

larityi.

The. comparison, which. governs thexpolarity oifV the tpulse output; is between .thef intelligence'-` Waveformand the integrated A.amplitudesof alli precedinggoutput `pulses,.atth'e.timeeoi the .transf missionofagiven pulse. When Aone islthe bigger a..pu1se of. one `polarity is transmitted and `when the .other is .the bigger, `thepulsenf fthe opposite polarity isA transmitted.A f

At the receiving end the `pulses are integrated.

and low pass ltered- .to recover. the. intelligence.

Waveform.

These .types of `modulation arehereinaiter col-` lectively referredVv to.. as pulse communication code BJ An embodiment of the invention Willilnow' be.`

described which illustratesa telephone exchange system viewed from -the standpoint of thetrans-4 mission .problems only, the selective features;

which are not material' totheinvention being omitted for simplicity. Pulse 'time' modulation isused in one part of a transmission'v system-.and pulse code modulation'A irranother part;

A number of physcal channels:serving'inL dividual subscribers arearranged'- in" groups, each group being takentoa sub-centre:

Fig.' 1 shows ten` suchA physical channels IV to|0;terminating"ina sub-centre II. Altogether` tensuch groupsof channels andisubl-centres are catered for in this embodiment,` only thearst and' the last being shown in the `figure, the---tentl'r sub-centre beingj indicated by' `reference numeral- I 2'.l

Each sub-centreA comprises a distributorl def-av vice adapted to receive the physical channels and distribute them in time in arecurring cycle of.-

time positions.

The` distributor applies thetimeedstrlbuted channels'to a single transmission path I 3- inthe form oli-pulses at-the-different time positions inf.` the said cycle andisi further adaptedto convert. intelligence carried by f the channels.` into timer modulation of the pulses respectively represents ing the channels, in the said cycle.

A` device-suitable asia distributorfforf a sub:- centre is ldescribed: inl co-ipending application; Ser. No. 777,818.

Reverting to Fig: l` oithe accompanying draw ings a numberof" incomingl transmissionvr paths such as I3, I4, I5 and I6, I1, I8, 9,201y 2|, and 22 arel brought 5to central transmittingv equipment; 231 and there terminated onI individual. coder-retimercircuits such.V as 24.1 Theffigurer show-s asystem: having `10 such incomingA transmission pathsand'. coder'rei',imers,-` thexrst, `2li;

being shown incomplete schematicuforin and.` the tenth in skeleton, thetintermcdiate coder.-

retimers being indicated byffanotc.

The functions of the coder: retimers.` arer to take `the time modulatedtime positionrmultlplex pulses .from the; ten transmission paths I3: to; 22 inclusive; to convert-the linear; time:- modulation: to la; modulation inftl1e1-ilicedc` amplitude-fle.velsiL and to'zscorivert` orfsteps' of@t pulse modulation.: A',

6?@ this step modulation` `in'toz-a: five; unit ccderfeacin unit of.: tlfieyccde:having".4 twopossible conditions represented: by` aJpulsepnabSence offaipulseirefi spectively.v This. canprovide: 32 r combinations. f i representing 32 fixedV amplitude levels..`

At the-same time :the Aivewcoded output.A pulses from` each. coder-retimerl` are.: concentrated.` in`v durationso that theWholefive-pulseperiod takes-1l approximately one tenthrof the'time. allott'edu to the original;channeliit` represents, andthe intervening a nine; tenths isf taken up with r similar fivefunit code pulses from. a1- correspondingchanf-` nel -.of eachofzthe nine` other channelgroups;-

Itlhasbeen statedsabovefthat. the.- time position:

\ cycles'ofgthefvarious `groupsware coincident. Scifanasifrequency is concerned; this is so, butifrorn* the'vpoint of View. of phase; the` position .iscmorer complex;Y e

The :frequencyfof: the time cycles: of ten distribu# tors-in.the;various-:transmitting subcentresbmustf be synchronised and this may be:l done by.` thc`V` transmission from .1 the s centralf. equipment; of..Y ai synchronisinggfrequency or alternatively; since-sini' aftwo way system'-the'reLWillfbeA a receivingsube-4 centre associated with each transmitting:v sub-*- centre which will receive on a" distributor time modulated pulses fromy central.receivingecluipA ment. associated With the` central transmitting-i equipment,` on a=timeposition cycle `of` the'same1 frequency: as Athat `of the transmitting` subcentres f and a; synchronising signal can-.be .obtainedifronie the received: time modulated cycle and'` used l tol synchronisef the distributorat the associated transmittingsubcentre.` Whatever r the relative phasespf thetime posis tions-cyclesxat the sub-centres, they will notre-- main the same after transmission overrthe differ- 4f entftransn'iission-A paths-.I3 to 22 :Whichrmay belfof diierentlengtlis andhavefdifferent delay times. l..

This :cannotxbe tolerated at 'the ycentral "trans= l mitting equipment and compensating` delayscir-v cuits; `must A.be inserted in the* 'individual trans'- missiontpathsto adjusttheir relativegphases' after-f theyfhave been upset byi the` different transmise sionzpaths; I

Forfreasonsexplained below; thisfphase adjust ment` is arranged sothat the-Stime positions of?` thewdiierent groups are staggered byarsmallv amount; thattisA to :say substantially 4byrthertime" 'l taken to transmit a live unit code-'fromf a` coder` retimerfin'the. central 4transmitting equipment.

This-is' about-fone` tenth of "the time allotted to each'time position-in agroup'so thattherepwill be a` substantialr overlap between channels `on the.

groups. Forqinstance-the `timeposition of chan'- nel I"of'groupBlwillstarta tent-h of a time-posi tion' duration later-than the'timeposition of ,chan-f' nel I lof group 1A.

The relative phasing of-the-various"distributors' atfthe `transmitting subcentres is thussunimpor tant provided that :it remains constant. This cani` be-ensured-by frequency synchronisation without regard to" phase, which isl fortunateL because ai phase synchronisingsignal emanating from. theT` centralequipmentWould'be likell7 to undergo `dif+ l ferent delaysr intransmission `tothe different subs4 centres Yandftn avoid* this; eachu groupJwouldgneed' a `phase correctingfdelay deviceV to compensatefor` the delay in the transmission path fromftheE cen@ tral equipment:

The `general A.principle i' of operation of rthe l coder:

retimersisasfollows: l

Each ofr them has: twoidentical. parts which function alternately: Oner part .scans `the 4pulse of@` onephannel on .its Loccurrence andfccdes, retimesf.-

7 and transmits-during the time position allotted to the next channel. The other part does the same but is displaced by one channel period so that one i part encodes the even channels and the other part the odd channels.

Each part of a coder-retimer has an input gate (2B, 21). These gates open alternately and stay openfor substantially the time position duration of the channel to lbe scanned.

The timing is arranged as follows:

The central transmitting equipment has a common busbar 42-carrying pulses ata repetitionv duration, so that output pulses ofthe frequency dividers of all the coder retimers are evenly spaced within the time position duration of a group channel.-

The output pulse from the frequency divider 28 is passed to a divide-by-two circuit 29 which diverts alternate pulses from the frequency divider alternately to outlet connections 30 and 3l. These pulses are elongated again by the divide-by-two circuit so that they have a duration equal to a group channel time position duration. These elongated pulses are phased so as to coincide with the time positions of the group served by the coder-retimer which have themselves been phase-adjusted in the manner explained above.

The gates of the two parts of acoder-retimer are alternately opened to admit the passage of alternate channel pulses of the group served by the coder-retimer.

A suitable circuit for the frequency divider would be ya conventional gaseous ldischarge tube `counting train with an output -circuit connected `to a particular ldisch-arge gap in the train. Starting from rest the counting would commence .at the beginning of the train and fan output pulse Would'be drived from the output circuit connecferent gaps to the ou-tput circuit in the frequency dividers of the different coder-retimers, the output Ipulse could be made to occur at diierent times for the different coder-retimers, as required. In co-pending applica-tion |Ser. No. 763,655,` iiled July 25, 1947, there is described a multi-gap discharge tube adapted to i'lre its gaps in succession when supplied with recurring pulses, -due to ionisation coupling from a fdischarging gap to an adjacent unred gap to eX- tinguish its gaps when :all have fired and to repeat the process without pause so long as the pulses are applied. One lform of this device h-as its gaps separately'connected so that individual circuits'can be inserted in series with any gap. Such a device operates as a counting train and the connection of an output circuit in series with a chosen gap would enable it to fulll the function of frequency divider 28. i

The divide-by-two circuit 29 could be a double' one to the other each time a pulse was applied to it. Such a circuit would give 'an output to one bar 36.

The gate openingV pulses are provided by the connection .'(30) in one stable state and to another connection (3|) in its other stable state, and these` pulses would' have the required duration and phase for opening gate circuits 2t and 21. v

4liiach part of a -coder-retimer has 'a coder (32,

33) fed by an input gate (26, 21 Yrespectively).

These coders observe the modulation of the pulses admitted to them by their respective input gates 'and count its modulation time displacement in Y relation to a reference time. The count is su-ch that 32 predetermined :time periods are together equal to the longest possible time displacement corresponding to the peaks of modulation. The count is to the nearest whole number of such predetermined time periods below the time of modulation displacement of lthe scanned channel pulse. Each coder feeds an output gate (34, 35) which has five sections. The said count, by a coder, is 4preferably carried out by a binary' counter having iive stages each stage having two states which may be considered as the on and off states respectively. When a count is ycomplete, any given number of the predetermined time periods resulting from the count results in a particular combination |of settings of the individual stages of the binary counter. The five sections of the output gate are set by the five stages of the binary -counter and when an output gate opening pulse is `applied to all, those set by a bin-ary coun-ter stage in the on condition are opened, and the others not.

A recurring coding pulse from `one of ve common busbars is appliedcontinuously to each of the output gate sections so thatthe pulses fed to the ve sections are in a different phase from one another. These pulses are admitted by those gate sections which are opened and blocked by those which are not. A common circuit, Yto which the outputs of the ve output gate sections Iare applied, thus receives a ve unit code representing the amplitude of the scanned channel pulse to the nearest lower amplitude of 32 xed amplitude levels or steps. This only happens during the output and gate opening pulse. The determination of the levels is thus done by the coder and-the ve unit coding the output gate.

Both output gates of a coder-retimer alternately feed their output linto .a common busbar 36, and the timing of the coding pulses applied to the five output gate units, together with that of the gate opening pulse determines the time Aduring which a given channel appears on the busdivide-by-two circuit 29, the connection supplying the coder of one part of a -coder-retimer, being'used to provide the output gate opening pulse for the other part in a manner to be explained later.

The circuit of Fig. 3 which may be used as a coder, in this embodiment, requires Ias an input, a reference pulse occurring `on or after the opening of the input gate to the coder, and a pulse iderived from the modulated channel pulse to be scanned, the time between the two being measured. The referencepulse mus-t precede the derived channel pulse in the earliest modulation timing 0f the latter.

Itis convenient to derive the reference pulse ferred way however is to use the leading edge of the input gate pulse totrigger a pulse shaping pulse.

. and 53. Alternatively these ve busbars'may be supplied from an independent source.

f "(9) shows the'waveform of the input gate Qopening pulses applied over connection 30 to input gate 26 (the corresponding pulse in connec- I vtion3l Yis shown at (r) Y ,.(h), shows the pulses from pulse shaper v3l 4applied over connection 40 t0 diiferentiator 38.

j(i`) shows pulse time modulated pulses of alternate channels, from transmission path I3 lpassed through input gate 26 the other alternate channel pulses are shown at (t) k(j) shows the output of differenator 38 consisting of a negative peak derived from the trail- -Aing'edge of the pulse shown at (h) followed by j another negative peak derived from the trailing edge-of the timepmodulated channel pulse shown at (i), and this latter peak is varied in its timing according to the modulation of the channel These peaks form the input to coder 32 and the time between them is counted to the nearest lower whole-number out of the 32 predetermined time intervals.

(k) shows the pulse applied by pulse shaper 3l along connection 4l to, differentiator 39 and Valsoto output gate 35 as the output gate opening pulse for that gate.

, (Z), (m), (n), (o) and (p) `show the Vfive pulses y applied from the iive busbars 43, 44, 45, 46 and .41 to the iive sections of output gate 34 respectively. f must be fullled for a section of an output gate .to transmit a pulse: The corresponding binary As previously explained, three conditions counter stage must have been set to the on condition by the coder count, the output gate opening pulse from pulse Shaper 3l must be present and a pulse from the appropriate one of the jbusbars 43 to 4T must be present.

The latter pulses are applied all the time but canfonly be effective during the output gate opening pulse and so (l), (m), (n), (o), and (p) only show the effective pulses from busbars 43 to 47.

(q) shows a pulse derived from the trailing .edge f the pulse shaper 31 pulse which is used toV clear downV the binary counter in the coder after it has done its work of delivery of the code to bus bar 36. This pulse may be taken from connection 4| as shown in the gure, the trailing edge of the output gate opening pulse being differentiated by differentiators 54 and 55 for vthe two parts of a coder retimer.

(rk), (s), (t), (u), (o), (w), (y), (aa) and (ab) show the corresponding waveforms in respectof the other part of coder-retimer 24.V

(ac), (ad), (ae), (af),.(ag), (ab), (ai), (ci),

E. (z]c)and (al) show'rthe timings of the various channel code pulses from channels of the various kgroup/s, as applied to the busbar 36. All ve pulses areshown aspresent in each case but in fact of course some pulses will usually be absent according. to the particular code combinations Y set up by the coders.

(am), (an) and (o) show respectively the pulses from frequency divider 28 and from pulse shaper 31 along connections 40 and 4|.

(ap), (aq) and (ar) show corresponding wave forinsY for the coder-retimer of group B.

(as), (at) and (au) show corresponding wave f forms vfor the coder retimer of group C.

i VSimilar wave forms with corresponding phase These last nine graphs serve The coders and output gates may be substantially of a type described in U. S. PatentNo. 2,272,070, issued February 3, 1942, and shown in Fig. 4 of the drawings accompanying that patent. The circuits as described in that patent may be simplified somewhat and to indicate a suitable form which they may take, Fig.A 3 is a reproductionA ofl Fig. 4.0i the drawings accompanying the said patent, with the simplifications made. yFor ease of comparison with that patent the same reference characters have beenused in Fig. 3 as were used in Fig. 4 accompanying the said patent. v Y

vThe following is a description'of the ofthe simplified circuits. v Y

`The circuits will -be described as if they were acting in the role of coder 32v and output gate 34. Terminals on the figure aremarked with lower case letter references indicating that waveforms as indicated'by the same letter references in Fig. 2, are applied to such terminals.

lThe valve AT is a double triode connected as an Eccles-Jordan circuit having two stable states, to which are applied at-BT pulses from differentiator 38, that is to say a reference pulse followed by a pulse whose timing is varied according to the modulation impressed upon the channel pulse being scanned. The reference pulse triggers AT to conduction on the left hand side and the succeeding Vderived channel pulse transfersV conduction to the other side whichis the rest condition of AT. VA connection from the grid GT of theleft hand side of AT is taken to a pair of grids of ay double triode ET which is normally cut-off on these grids but is opened while operation GT is positive and theleft sideof VAT is conducting. This controls a counting operation which counts the `time between the reference pulse and derived channel pulse, which'is hereinafter referred to as the modulation interval.

The counting operation consists of counting the number of complete cycles of an oscillator DT which occur in the'modulation interval, by means of a iive stage binary counter consisting of the double triode valves Cl, C2, C3, C4, and C5 each connected 'as an Eccles-Jordan circuit. The oscillator DT operates ata frequency such that 32 complete cycles occurV during the longest time between the reference pulse and the derived channel pulse delayedto the extent of peak -signal amplitude at ,100% `modulation. This Vperiods of the grid GT of AT. With modulation any number of cycles 'of DT'between 0 and 32 can be passed to point FT during a modulation interval, according to the speech potential at a specified moment. 'For instance if C4 has changed over to its second condition of equilibrium for the first time, the number of cycles of DT which have beenY transmitted is eight.V There is thus aparticular combination of positions. of` final equilibrium .fori thewari'ous counterstages `correspond-ing to; anyl number.' of cycles transmitted between zero and. 32; .It will be seen therefore that `the counting .process assesses the amplitude ofthe modulating wave# form in predetermined steps.. Thev Voir condi. tion of the counter tubes is with the left hand side conducting, which is also the rest conditionof the tubes and the on" condition is with the right hand side conducting. i

The output gate consistsl of ve pentodes PI, P2, P3, P4 and P5' each of Vwhich'has'a connectionto its suppression grid. from'` the right hand grid of one of the valves CI, C2, C3, C4 and C5v and these suppressors are normally biassed so as to cut off anode current through the gate valves, but are changed so 'as to .permit anode current when the corresponding counter 'tube is in the number 2 or on" condition. The screen grids of 4the .pentodes are connected to theter-` minal marked (lc) to which the output gate open# ing pulses, as shownin graph (K) of Fig. 2, .are applied. These `screen grids are normally. cutoff and are only opened during theloutput gate opening pulses. Each of the pentodesl has itslcontrol grid `connected .to `a separate terminal and each of these terminals is connected to one of busbars 43 to 41. These controlv grids. are normally cut-oli and are only opened during` a pulse inthe corresponding one of busbars 43 to 41.

It is thus seen thateach of thepentodes Pl toSPS will only pass anode currentwhen three conditions are fulfilled namelythe voutput gate pulse must'be present; a'pulse from the corresponding one of busbars `43 to l41 mustibeipresent; the associated counter stage must be in th'e.on.`

condition.'

Each pentode has the primary'winding. of a transformer in `its `anode circuit land the sec.- ondary windings of these transformers are connected in series and to terminals (ac) to which a pulse is applied when anode current ows in one of the pentodes.

The net result is therefore that a five vunit code is passed from terminals (ac) to 'busbar' 36 (Fig. 1) to` which they are connected, the over-all timing being determined by the output gate opening pulse appliedV to (K), the' timing of the individual code units being determined bythe timing of the pulses in-busbars 43' to 41 and the presence or absence of'a pulse at' each of' the code unit time positions being determined by the state of the Iive counting stages as a result of the count of cycles of DT during the modue lation interval.

To reset the `counter stages of the "coder in preparation for scanning the nextalternate channel timeemodulated pulse a restoring pulse asshown in graph (q)v of Fig. 2 is applied to the terminal marked (q) whence it passes through decoupling condensers and resistances marked T to the right hand grids of the five counter stages.`

The restoring pulse isa negative `pulse and restores conductionto the left hand side of each counter tube. This condition is the same as in the original conditions 'of the Whole circuit, which consequently is ready to scan another modulation interval.

The transmission path to which busbar 36 of Fig. 1 is connected, leads to fthe receivingend shown `in Fig. .4, where' there. is central .receiving equipment.

This comprisesbusbars. similarrto thoseat the central transmitting equipments which` rior. ease- 14 of ,comparisonV Vare merals.:

The busbars 42 and 443` to'41 must. befsyn.-v chronised in frequency and phase. withthe .pulses received overV theV transmission path connected to busbar 36 in Fig. 4. i

There Iare many ways of doing this, onev being to reserve a certaintime out of the time cycle for the transmission of a -synchronising pulse of different amplitudes or width` from thecode pulses. A channel might have tobe devotedpto this or else the time cycle could be divided-.into such periods for. the channels as to leavewavtime at the end of a cycle less than the. time'requirecl for a complete channel.

Thispulselwould be passed througha'discrime inator 56V to pick out the pulse from 'othersfin busbars 36, to the pulse generator, resprmsible,- for producing the pulses in busbarvdz and would synchronise this oscillator. All. otheru timings; at the receiver beingderived from-yor: synchro'- with the incoming code pulses in bus barf36-(1ig.A 4). In a practical telephone exchange system the central receiving andthe. central transmit-` ting equipmentwill be-parts of one exchange, so th-at corresponding busbars ofthe `two -canile supplied from `a commonsource. Thereceiving. equipment busbar supplies-must be inzsynchro-` nism, however, with the pulsesreceivedoverfthe common transmission path andmust bev delayed as compared with the transmission. equipment busbar supplies, by aI degree equalto therdelay time ofthe common transmission path.. There' is thus a good-reason .for synchronising -the .res

ceiving equipment bus .bars with thoseot the transmission equipment bytsynchronisingasignals conveyedby the commontransmission path in La manner such as .that above described..-A

The incoming code. pulses. must beldivided up into channel groups each of iive codetimeposi tionsand each channel group-passed-to adef. coder retimer. A decoder retrner 58 lis shown for group A in complete schematictform..A The decoder retimer for group J is shown in skeleton and marked 59,V and the-other decoder retimers for groups BAtoAI are omitted l to simplify'...the. drawing.

The process of selecting the rcode pulses. of: a group consists of. taking `every` tenth. group .of: code pulses vinthe complete cycle so thatf the basic pulse .code .group repetition. frequency A(in busbar 36) which is the same as the frequency. ofthe pulses in bus .bar 42,..needs.to. be .divided by ten by a frequency divider lisimilar toirequency divider 28 in Fig. 1. timer has a frequency `divider and all are displaced in' .ph-ase by one complete. vchannel-time period of the` cycle of bus .bar 36.` ('I'his period is one micro-second in the examplequoteds-)Q A pulse derived` from thefrequency dividerlll; and called the receive gate opening. pulsa-"is. used to open five gates 6I to 65 at the timings-of. the channels of the group` in question.

The ve gates also have connections tovindividual ones of busbars43 to I41andare barred.. except when a pulse is present in the associated. busbar. In addition lthe `code pulses vfrom the :busbar 36 are applied in common `tothe-livres gates which are` still barredin the absence ot` a code pulse from busbar 36 during thecoincidene" of a rreceivegate opening pulse: and Ia `.pulse-front the associated busbar 43to 41.

VEach of -the ve gates isconnected'sofasiftd set a stage of a five stage binlwfqunllentmat 'given the same referclrniie Each decoder N.re--r five code pulses of a channel, in busbar 36 (i. e;

one microseoond in the example quoted).

It is the task of a decoder retimer to restoreA the channels of its group to pulse time modulation and to distribute them evenly within the cycle period of the individual group (100 microseconds in the example quoted of ten channels at a cycle recurrence frequency of 10 kc./s., each channel having one tenth of the period, i. e. ten microseconds, allotted to it). The channel period of the group cycle is devoted, as to one tenth, to the reception of the code setting from busbar 36. The remaining 9 tenths are therefore available for assessing the code and marking it out to a pulse generator which will produce a pulse modulated in time according to the modulation information carried by the code.

As this pulse generator can be independent of the code-assessing apparatus the time taken by its pulse need not be accommodated within the remaining nine tenths of the channel period in the group cycle. Y

In the example quoted this nine tenths period is 9 microseconds long so that, leaving one microsecond as an additional margin of safety against inter-channel cross talk (the code reception period also forms a safety margin in the group cycle), the assessing process can be timed to take 8 microseconds, i. e. 4/5 of the channel period in the'group cycle. The assessment of the code is achieved by a ve stage binary counter similar to a coder in Fig. 1, which is set to a certain condition by the ve gates and then almost immediately pulsed till it reaches its rest condition. Thus with 32 possible conditions, the number of impulses required to drive it to the all-off condition is the complement to 32 of the number represented by the'code. A gate circuit is opened at the beginning of the count and closed at its end, and on closing triggers the said pulse generator to deliver a pulse of fixed duration to the group transmission path (61 to 16 of Fig. 4 are the ten transmission paths for the ten groups). The group transmission paths lead to ten receiving sub-centres two of which namely those of groups A and J are shown in the left of Fig. 4. Each subcentre has physical connections to the receiving apparatus of the substations of the group.

The maximum number of impulses is 32 and there is a period of 8 microseconds during which they must take place, in the example quoted. This demands an impulse rate of 4 megacycles which is the same as the frequency of oscillator DL of Fig. 3.

The gates and decoder of a decoder retimer may be substantially. of a type described in the said U. S. Patent No. 2,272,070 and shown in Fig. 5 of the drawings accompanying that patent. The circuits as described in that patent may be simpliiied somewhat and to indicate a suitable form which they may take, Fig. 5 of the drawings accompanying the present specification is a re production of Fig. 5 accompanying the said patent with the simplification made. For ease of comparison with that patent the same reference characters have been used in Fig. 5 as were used in Fig. 5 of the quoted patent.

In Fig. 5 there are five double triodes LCI to LC5- connected as Eccles-Jordan circuits and functioning as a five stage binary counter as in Fig. 3. In this case however the condition of rest is with the right hand side of the double triodes conducting. Y V

I Five `pentodes BIto B5 are the five gates IIv to of a decoder retimer and their suppressor grids are connected to terminals (b), (c), (d),

(e) and (,f) each leading to one ofbusbars 43 to 41 (Fig. 4). Their screen grids are connected in common to a terminal (K) to which are applied the receive gate opening pulses. Their control grids are connected to a termina1 marked 36 signifying that it is connected to busbar 36 of Fig. 4.

All these grids are normally biassed so that each will cut-off the anode current of the valve irrespective of the condition of the other.

When a channel of the appropriate group has its codepulses present in busbar 35, the pentodes will be "opened together on their screen grids and in turn opened on. their suppressorA grids in step with the iive pulse time position of the code, by the pulses of busbars 43 to 41 (Fig. 4) and opened or closed on their control grids according to the presence or absence of a code pulse at the time when each is opened on its suppressor grid. Only a tubeopened on all three grids will pass anode current and in doing so will pass a pulse from its anode to one fof the tubes LCI to LCE with which it is assoelated.

The pulses from the anodes of BI to B5 are applied the the left hand grids of LCI to L05 respectively and a pulse present will change over the stage concerned, into the on condition.

' At the end of the receive gate opening pulse the ydiiferentiator connected to terminal K, passes a peaked pulse to the right hand grid of a double triode HL connected as an Eccles-Jordan circuit with its right hand side normally conducting.

The right hand side of HL is cut-off by the negative pulse and a positive pulse is passed from the left hand grid, to the lower grids of a double triode EL which is connected up as a push pull amplier of oscillations produced by oscillator DL. EL is normally cut-off by its lower grids and on receipt of the positive potential from the left hand grid of HL it becomes conducting and passes on the oscillations of DL to the first stage LCI, of the binary counter LCI to LCE.

The binary counter, starting from the position to which it is set by BI to B5 completes its cycle up to the point where all the stages are restored to the ofi position. The number of impulses taken to do this is the complement to 32 of the number represented by the setting of the counter by BI to B5.

rIhe nal act in the cycle'is the restoration of LCS to the olf condition which passes a pulse to the right hand grid to HL to restore conduction to the right hand side, rendering the anode less positive. This cuts off EL and stops the counter. The left hand grid of HL is also connected through a rectifier i3 and a negative pulse is passed to pulse generator S0 which is triggered thereby and produces a pulse of fixed duration which may be two microseconds in the example quoted. This pulse is modulated in time in relation to the puise from diierentiator 'I7 which is of xed timing, by the time taken to count the binary counter back to the all off condition.

To understand the operation of the counter the better it is useful to consider the basic operation of a five stage binary counter.

If impulses are applied to the rst stage, they Every other impulse being the first stage backinto the ofi conditionA and it is the act moving into the ofi condition by one stage which Ychanges the condition of the succeeding stage.

Every alternate impulse therefore changes over the second stage which moves back into the off position every fourth impulse.

`Every fourth impulse changes over the third stage which therefore moves back into the oif" condition every eighth impulse.

Every eighth impulse changes over the fourth stage which therefore changesback into the oif condition every sixteenth impulse.

Every sixteenth impulse changes over the fifth stage which therefore changes back into the off condition every thirty-second impulse.

Whatever the initial condition of the stages, the application of 32 impulses brings` them back into the same condition. Y e

`EJt-arting with all stages off, 32 impulses will bring the small back into the off condition. During this cycle the fifth stage will change over. to the on condition at the sixteenth impulse and back to the off condition at the. thirty second impulse, thatis to say, onechange over to "on and `back to off in the complete cycle.

If the stages are set articially (that is to say other than by application of impulses to the first) to conditions representing a certain number,

that is to say into the conditions they would on and the 32nd sees it move. back to 'oit.`

Any setting more than sixteen sees it already at on and the 32nd condition sees it move back to cih The move back tov off by the 5th stage. can therefore be usedas' an indication that all stages are restored to off by using this change over to trigger a circuit cutting oi the, impulses in the same manner as other stages cause the change to theirvneighbours when they change over from on to off. q

When a counter is setartiiicially to a certain condition and then impulsedbackto the all offv condition, the number of impulses required is the complement to 32 of the numberrepresented by the artificial setting. In` some applications this complement has to be re-inverted tofgivev a direct indication of the number represented by the artificial setting, but where the setting represents a modulation' the complement can give the required answer without re-inversion, the result being simply to change the phase of the modulationV through 180. If this was of any momentin any part of a circuit it could be `dealt withby changing over two conductor'connections,

In a modification lofthe described embodiment the coder-retimers of Fig. 1 are simplifiedso as to have only one part andto take the form shown in Fig. 6, the same numerals being used as in Fig. 1 to denote Correspondingitems.`

The operation of this onepart coder-retimer is best appreciated by taking the frequencies of the example above quoted.

The time, in the transmission path fror'na subcentre to the central l transmitting equipment, allotted for the accommodation of a channel, is

18 tenmicroseeonds, during which thek modulation of the Vchannel pulsemmusij.V be assessed and `en-V coded and vthe coder cleared down in readiness for theA next channel.

.i is used for a channel pulse in the group transe mission path. Thisleaves an interval ofltwo microsecondsas the minimum interpulsefinterval between channels which is a desirable factor of safety against interchannel cross talk` During this two microsecond interval the one microsecondsoutput gate opening pulse must take place, and it maybe timed to take plaeein the middleof the two microsecond interval, leaving 1/2 a microsecond forthe stopping of the: counter and the opening of the gates, and 1/2 a microsecond after transmission of the five unit code, for the clear down of the counter and the gates prior to the next channel, which commences with aV reference pulse.

The frequency-divider 28 of the onepart coderretimer may deliver atrigger pulse at the beginning of each channel time period of the group cycle, This trigger pulse may be used to trigger apulse Shaper 82 comprising a self-restoring iiipflop circuit with a restoring period of 8 microseconds (where this is the period chosenfor the accommodationof the modulated channel pulse) The output ofthe flip-flop circuit is'differentiated by differentiator t3 to givea positive peak at the beginning and a* negative peak at the end ofthe 8 microseconds recovery period of `the flip-flop. The two peaks are separated by rectiers and passed along` separate conductors 84, 85 respe'c tively. Y

The positive peak is passed'to terminals BT in the counter to act as the reference pulse which starts the eountand the; channel pulse passes through diiferentiator 38 where its differentiated trailing edge also passes to terminals BY to stop the count. r

`The negative peakk in conductor 85 ispassed through a 1A? microsecond delay circuit 86 to an output gfategopening pulse generator 8l which delivers a'pulse to the output gate to open it for' transmission of the code. rIhe same pulse is passed through differentiator 88 which produces a peak at the trailing edge of the-pulse which acts as the restore pulse passed to the coder (being applied to the terminal marked (g) in Fig. 3).

There is thus no need for an input gai?, Since all channels pass Vin turn to the coder and not every other channel as in the arrangement of Fig- 1.

Therwaveforms at various parts of the Fig. 6 arrangement are shown in the graphs of Fig. 7, graphs (c), (b), (c), (d). (e), (f), of Fig. 2 being re-drawn to show the time scale.

Graph (au) `shows the trigger pulse from frequency divider 28.

Graph (aiu) shows the 8 microsecond pulse from pulse shaper 82.

Graph (am) shows the modulated channel pulse.

Graph (ay) Shows the positive peak from differentiator 83, which starts the count.

Graph (az) shows the differentiatedv trailing edge-of the channel pulse from differentiator 38.

Graph (za) shows the delayed negativepeak from 1/2 microsecond delay circuits 86, which triggers output gate opening pulsev generator 8l, to provide the output gate opening pulse shown in graph (ab). Y

Graph (ce) `shows the timings of the five code elements transmitted to busbar 36 and graph (ad) shows the restore pulse applied by differentiator 88 to the terminal markedq) on Fig. 3.

An important aspect of pulse code modulation A, deserves mention.

When constant amplitude steps are used for the assessment of the instantaneous value of the signal, signals of low level, such as quiet speech, have fewer effective steps representing the difference between peaksandtroughs of the signal waveform than is the case with high level signals such as loud speech.

As the amount of distortion introduced by the assessment of amplitude in vsteps is increased when few steps are used and reduced when more steps areused, it follows that low level signals will be considerably distorted. It has been'found necessary to overcomel this by grading the size of thevsteps so that smaller steps are usedvin the low amplitude range and larger steps in the high amplitude range. K

Theeifect of this is to exaggerate the periodic amplitude changes Vat low levels of signal as compared with those at high levels of signal. If this were to be translated back into sounds without reconversion, the result would be raise the level of low level signals and lower the levelroI^ high level signals, that is` to vsay-to reduce the dynamic rangepof the signal sounds or in other words to produce volume compression, which is sometimes called companding v The same effect may be produced, and in a simpler manner by applying companding to the signals directly before or in the process of translating them to pulse time modulation at the subcentres rather than to modify the amplitude steps at the encoding stage.

In theV embodiment described, companders (not shown) would preferablybeinserted somewherein the channel physical circuits I to I in Fig. l; Alternatively, using the distributor described in relation to Fig. 3 of the drawings accompanying the said cci-pending application Ser. No. 777,818, filed October 3, V1947, a suitable shaping of the saidsaw tooth waveform could be used to give a companding effect in the process of connecting audio to pulse time modulation.

To restore the dynamic range at the receiving end, complementary expanding circuits with characteristics the inverse of the compander characteristics would be inserted somewhere in the physical circuits between the receiving subcentres and the respective channel receiving equipment.

The embodiment described above illustrates one combination of modulation methods. There are other possible combinations however as previously indicated.

It is possible, for instance to use pulse code modulation B in place of pulse code modulation A at and between the central Vtransmitting and receiving equipments with pulse time modulation between the substations and the central transmitting and receiving equipments respectively.

With this scheme it becomes necessary to have one coder per channel since the amplitude sample taken at one appearance of a channel must be quency dividers, one per channel,- could be usedl at the termination lof each group transmission path to segregatethe pulses of the channels and pass them to the coders. y 4

The coders proposed in the said io-pending applications No. 778,662 andNo. 77-8,663,`;led

October 8, 1947, require an input'intheform of continuous audio frequencyfwhich inV the first embodiments, in that application isdiiierentiated continuously and applied to two valvejjcircuits 4which respond to negative and positiv/echanges respectively in synchronismwith a local!master pulse input at a frequency equaltothechosen sampling speed. The same effectcan be obtained by the differentiation of an amplitude modulated pulse input whichcan be derived readily from the time modulatedchannel pulses, by known means.

The second embodiment compares a continuous audio frequency input with the integratedoutput of the coder and an audio input is essential for this pulse. Y Y Y Y The interleaving of channels inthe transmission path fed by busbar 36 may be achieved by the appropriate phase displacement of the pulse output of the coder by feeding all codersfrom a common master pulse source and timinggtheiroutputs with individual delay circuits. y

At the central receiving equipment, the pulses from busbar 36 are again distributed to` individual channel decoderscwhich restore the original signal intelligence by integration and low pass ltering of the channel code pulses or else, instead'of producing an audio frequency output, a periodic amplitude modulated pulse can be obtained direct by omission of the low pass filter and this may be converted by known means to pulse time modulation which by suitable phasing of the outputs from the various decoders may be combined on the group multiplexV cycle.-

Alternativelyjthe Vdistributor described inthe said co-pending application Ser. No.778,818, led October 3, 1947, and shown in Fig. 3Y ofthe `drawings accompanying that application could be used to distribute the channels of a`group on to the group multiplex `time cycle and?4 inf this case the low pass filters could be left inV situ andthe audio frequency outputs converted intoY pulse time modulated' pulses, distributedas required, and applied to -thetransmission Vpath Vserving the group receiving subcentre. i' f Yet another alternative combination 'ofmodulation methods is tovcombine pulse amplitude or width modulation in the transmission paths between the sending and'receiving' subcentres and the central transmitting and' receiving equipments respectively, with pulse code modulation A 012B at'and between the centrali transmitting and receiving equipments. 1

Yet another alternative is to assess the 4amplitudes of the channel signals on entry; to a subcentre in relation to fixed amplitude.. steps and transmit them'in the group multiplex cycle in the form of timewi,dth Vor ampltiude modulated pulses which will have fixed steps of modulation, for encoding at the central transmitting equipment the converse procedure :being ffollowed-at the'receiving'end. Q' 'j A device for assessing the -amplitude insteps is described in the saidlUCS. Patent v2,272,070 and 21 shown in Fig-1 of the; drawings accompanying that patent and this device could be used at the subcentre end ofthe channels for the purpose, theresult being used to modulate the group multiplex pulses.

This scheme would have the advantage of noise and crosstalk suppression inherent in the step type of intelligence modulation relation, for the benefit of the group transmission paths and thoughfthe coder shown in Fig. 3 ofthe drawings n accompanying this specification does not need to `be fed with a` step modulated input, it is equally well able to operate `from such an input as from a linear modulation input. The advantage of the coding is then added to that of step modulation forthe avoidance of interference in the busbar 36.

The converse process would be adopted at the receiving end. Indeed this is done in any event in the embodiment described since the outgoing modulation derived from the decoders is in step form and need nctbe `converted to continuous intelligence till it reaches `the .channel receiving equipment. Yet another alternative combinationof modulation methods concerns the use of Adifferent systems of iixed levels of the step modulation process of pulse code modulation A or of fixed minima of rising or falling signal amplitude between sucsive sampling instants, of pulse code modulation B in its first form 4for dierent purposes and/or indifferent parts of a system. This would be advantageous where different classes ci intelligence are catered for in the system. For instance, in the case of intelligence for purely signalling purposes a higher degree of inherent distortion could be tolerated than for speech intelligence and in a system such as that of the embodiment described `a channel or channels may be wholly devoted to this class of intelligence and, where the assessment of fixed levelsonxed level diierences (of 'pulse code modulationV A or B respectively) is carried outat subcentres, the levels could be assessed on a coarser scale of steps or a larger minimum level differentV respectively; In the former case the number of code elements required to represent the coarser steps is reduced and less channel space is required on busbar 3S which may be utilised to carry two signalling channels in the time taken by a single `speech channel.

Yet Vanotheralternative combination of modulation methods is the use of pulse code modulation (B) with one rate of sampling in one part cfr-the system and another rate of sampling in another part ofthe system.

In complex modulation systems thereare two aspects of the problem which can be distinguished. Onl the one hand there is the manner of pulse variation which is used to indicate varying states of the intelligence to be conveyed. For instance, in pulse time modulation, the phase or timing of a pulse is varied in relation to a mean or reference timing, and in pulse amplitude modulation, the amplitude of the pulse is varied.

`In pulse code modulation A, however, aseries of pulses istreated as a coding unityand, in the simplest form the individual pulses are transmitted or suppressed in accordance with acode, whilst in pulse code modulation B a single pulse of uniform amplitude is suppressed-and/or changed in polarity. The code modulation systems are `distinguished from the other forms of modulation previously instanced` above in that only a limited rangeof different conditions is catered for whilst in those other forms of lmodulation an innite gradation ci conditions between the extreme limits of modulation, is attempted, even though the attempt may in practice be frustrated by imperfectionsin the equipment and the transmission medium. H l 'y 7 V The manner ciA pulse variation viewed from this aspect is hereinafter referred to as the way of modulationj Onthe other hand, the` otheraspect of the problem concerns the relationshiplbetween variation of the intelligence to be conveyed and the modulation intended to represent it. Here again the modulation may attempt to follow with infinite accuracy the variations of the intelligence waveform or the degree of accuracy may be deliberately curtailed. Y, l n i y For instance, with pulse time, pulse amplitude, or pulse width modulation directly by audio frequency intelligence thel modulation of the pulse isfusually made to follow as nearly as possible, the instantaneous amplitudeo-f the intelligence waveform at the instants of sampling or observing this waveform throughoutlan infinite range of values between maximum andminimum.

With the step modulation process of pulse code modulation A, a `deliberate inaccuracy is introduced by defining the instantaneous amplitude of the intelligence to the nearest of the fixed levels (or the next level above, or thenext levelbelow the intelligence amplitude) Y In pulse code modulation B, `in its firstform another typeof deliberate `inaccuracy is introduced byignoring changes in.` intelligence waveform amplitude between one sampling instant and another, where such changes are less that of a xed minimum and where the changes exceed the iixed minimum byrecognising merely the fact that they do exceed such minimum without regard to the degree of the excess,.whilst in the second form of pulse code modulation B the modulationlis only able to `deflnethe rate of change of signal amplitude by `means of `a fixed rate of change of a discontinuous nature (i. e. in steps of uniform "rise and tread) a slow rate ,Y of change of signal amplitudebeing represented by a change in the fixed ratein one direction, periodically corrected by changes at the fixed rate in the other direction, and a rapid rate of rise being represented by a change at the fixed rate continued for longer thanthe persistence of the rapid rate of change of thelsignal amplitude.

The relationship between the variations of the intelligence to be conveyed and the modulation of pulses, viewed from this aspect. is hereinafter referred to as the "intelligence-modulation relation.

it should of course be noted that any translation of a continuous intelligence waveform to a series of. discrete modulated pulses constitutes a deliberate departure `from complete accuracy ofrepresentation of the intelligence waveform but as this factor is common to all forms of pulse transmission with which the invention is concerned it is not intended to be embraced by the term "intelligence-modulation relation.

To enlarge upon the distinction between the way of modulation and the intelligence-modulation relation some aspects of pulse code modulation A may be considered.

The intelligence modulation relation inherent in pulse code modulation A may vary in degree according to the number `of fixed levels 4chosen and according to the spacing of those levels on the amplitude scale. This spacing 'may `be graded so that instead-of the steps being"equ'ahthey` are 23 smaller in some ranges of amplitude than in other ranges.

The use of the stepped method of assessing the amplitude of the intelligence waveform has advantages of its own independent of those accruing from the use of a coded way of modulation. Amongst these advantages is the elimination of noise the amplitude of which is less than half the step interval and it may well be of advantage to reap these benefits by themselves in some parts of a system where the complication of a coded way of modulation is undesirable.

Again the way of modulation of pulse code modulation A can be varied both in the form of the code and the method of its transmission.

The number of units in a code combination can be varied from the number of used in the embodiment described, a lesser number enabling less precise intelligence to be accommodated'in a smaller frequency band-width and a larger' number enabling greater precision in conveyance of the intelligence at the expense of occupying a Wide frequency band.

In the embodiment described, the pulses of the ve unit code are transmitted successively on a single transmission path but they could be transmitted simultaneously on individual transmission paths, with minor modifications to the equipment described.

1n the central transmitting equipment the busbars 43 to 41 would be omitted and ve busbars connected to five transmission paths would be substituted for busbar 36. Each section of a coder-retimer output gate would deliver its output to one of these five new busbars and would be permitted to do so during an output gate pulse which could be of one iifth the duration of the pulse described. In the central receiving equipment the same busbar modications would be made and each of the i-lve busbars replacing busbar 36 Would be connected to one of the decoderretimer vgates and all would be opened simultaneously by a pulse from the frequency divider 60. A pulse on one of the ve new busbars could be used for synchronising the central transmitting and receiving equipments.

The code pulses in the embodiment described are live in number and each can have two alternative conditions, namely pulse present and pulse absent. If three alternative conditions were available such as the alternative positive pulse, negative pulse, no pulse, as used with pulse code modulation B, twenty-seven combinations could be obtained with three code elements and 8l combinations with four code elements.

The use of all these variations of pulse code modulation A is within the scope of the invention.

The description of the invention in respect of the use of group channels concentrated onto a common transmission path at a high channel repetition frequency, and the combination in one system of dilerent types of modulation has been framed so as to focus attention upon the transmission problems of Ha telephone exchange system using pulse technique and to omit reference to selection and interconnection problems with which the invention is not immediately concerned.

In a practical telephone system the arrangements of Figs. 1 and 4 would be amalgamated, each transmitting sub-centre having a companion receiving sub-centre and each incoming physical channel from a substation to the transmit- `ting sub-centre having a companion outgoing 24 physical channel from the associated receiving subcentre to the substation.

The central transmitting and receiving equipments are located together within an exchange, with their busbars 36 directly interconnected. The connection of a calling substation to a called substation would preferably be effected by transferring modulation intelligence from one channel (that of the caller) in each common busbar 3B to another channel (that of the callee), in response to designation signals from the calling substation. The answering signals of the callee could be similarly transferred from his channel to that of the caller or both Ways of communication could be carried by either the callers or the callees channel. I Y

The other combinations of modulation methods mentioned above would lend themselves to a variety of methods of ,selecting connections, for instance, when pulse code modulation B was used in central parts of the system the separate channels would be separated out from the group multiplex cycles to individual coding circuits Whose timing of code delivery could be directly controlled by signals present in the channel modulation.

rlelephone exchange systems of this general type are described in co-pending applications, Ser. No. 628,613 liled November 14, 1945, and Ser. No. 27,296. filed May 15, 1948.

While the principles of the invention have been described above in connection with specic examples and particular modifications thereof. it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What is claimed is:

1. In a telephone exchange system wherein communication channels take the form of time positions in a recurring cycle of time positions, the combination comprising a plurality of link circuits each having a recurring time position cycle, another link circuit having a recurring time position cycle, the time positions of which are shorter in duration and have a greater number than those of said plurality of links, and means for retiming and combining the time positionof said plurality of links to concentrate them in a single cycle of the time positions of said other link.

2. In a telephone exchange system wherein `communication channels take the form of time positions in a recurring cycle of time positions and intelligence is conveyed on such channels by electric pulses having at least one characteristic modulated in conformity with the said intelligence, the combination comprising a plurality of individual link circuits, each having a recurring cycle of time positions, the way of modulation of said pulses and the intelligencemodulation relation being different as between different links of said system, and means for transferring intelligence from one link to another and for converting the pulses from the Way of modulation and the intelligence-modulation relation of said one link to the way of modulation and the intelligence-modulation relation of said other link, that way of modulation and intelligence-,modulation relation being used in eachlink of the system which is best adapted to the transmission conditions of that link.

3. In a telephone system, the combination, as defined in claim 2, in which one of the links is moreprovneto cross-talks than the others, said link having -a Way ofmodulation and an intelli- `talk.thanethatof Vother ways of modulation'and ..intelligence-modulation .relations used in -other lihksi Y g4. rIn aftelephone excliangesystem, the combinaticnfasdened-in claim 2, in which links betweenlsiibscri-bers and an exchange have different systems of modulation'lfthan links operating within the exchange.

5. In a telephone system, the combination, as dened in claim 2, in which at least one of the different ways of modulation and intelligencemodulation relation is pulse code modulation A and the use of such modulation is confined to links which accommodate a larger number of time positions in a given time period than do other links of the system.

6. In a telephone exchange system, the combination, as dened in claim 2, in which at least one of the different ways of modulation and intelligence-modulation relation is pulse code lmodulation B and the use of such modulation is coniined to links which accommodate a larger number of time positions in a given time period than do other links.

'7. In a telephone system, the combination, as claimed in claim 2, in which in at least one link of the system the intelligence-modulation relation is of the type where the instantaneous amplitude of the intelligence waveform is dened relative to a scale of predetermined xed levels oi amplitude, and the way of modulation of pulses is in the form of a variation of the timing, am-

plitude or duration of the pulses, and in which in another link of the system the way of modulation is of the type where, of two or more time intervals, each is either occupied by the transmission of a pulse or not so occupied, according to a code of combinations and permutations, cach representing one of the levels of a scale oi iixed levels as aforesaid.

8. In a telephone exchange system,- the combination, as claimed in claim l in which intelligence is conveyed by electric pulses occurring during the said time positions, such pulses being modulated in conformity with the said intelligence, and in which the way of modulation used in the other link taking the form of the said :single cycle of time positions into which other cycles of time positions are concentrated, is that or, pulse code modulation A.

9. In a telephone exchange system, the combination as claimed in claim 1 in which intelligence is conveyed by electric pulses occurring during the said time positions, such pulses being modulated in conformity with the said intelligence, and in which the intelligence-modulation relation used in the other link taking the form of the said single cycle of time positions into which other cycles of time positions are concentrated, is that of pulse code modulation A.

1C'. In a telephone exchange system, the combination, as claimed in claim 1 in which intelligence is conveyed by electric pulses occurring during the. said time positions, such pulses being modulated in conformity with the said intelligence, and in which the Way of modulation used in the other link taking the form of the said single cycle of time positions into which other cycles or" time positions are concentrated, is that of pulse modulation B.

il. In a telephone exchange system, the combination, as claimed in claim 1 in which intelli- 4gencev is conveyed by electricpulses occurring during the said timepositions, such pulses being modulatedin'conformity with the said intelligence, and in which 'the intelligence-modulation relation used in theother .link taking the form of the said single cycle of time posit-ions into which'other cycles of `time positions Vare concentrated, is that of pulse code modulation B.

12. In a telephone exchange system, the combination, as claimed in claim 2 in which the intelligence-modulation relation in at least one link of the said system is of the type where the intelligence waveform is assessed at regular spaced instants and the change in amplitude between two consecutive instants measured in relation to a predetermined fixed minimum degree of change, the modulation recording three alternative conditions, namely, rst an increase in amplitude which exceeds the said measuring, second, a decrease which exceeds the said minimum and third no change or a change which does not exceed the said minimum, and in other links of the said system the Way of modulation is of the type where three alternatives namely, a positive pulse, a negative pulse or no pulse at all, at a certain time position in a recurring time position cycle, records three alternative states of the intelligence waveform.

13. In a telephone exchange system, the combination, as claimed in claim 2 in which intelligence-modulation relations based on scales of predetermined fixed amplitude levels are used in a plurality ofgdiierent links of the system, and in which diierent scales of such xed levels are used in diierent ones of the said plurality of links. I

14. In a telephone exchange system, the combination, as claimed in claim 2' in which the intelligence-modulation relation in one or more links of the said system is of the type where the rate of change of signal amplitude is `compared with a predetermined fixed rate of change of a discontinuous or stepped nature, the modulation recording two alternative conditions, the iirst when the amplitude of the intelligence waveform is above the amplitude of waveform conforming to the said xed rate of change, the second when the amplitude of the intelligence waveform is below that of the said waveform conforming to the said iixed rate of change, and other links of the said system the way of modulation is of the type where only two alternative modulation conditions of a pulse are possible.

l5. In a telephone exchange system, the comhination, as claimed in claim 2 in which in one or more links the way of modulation and intelligence-modulation relation is pulse code modulation A, and in one or more other links the way of modulation and intelligence-modulation relation is puise code modulation B.

16. In a telephone exchange system, the combination, as claimed in claim 2, in which the way of modulation and the intelligence-modulation relation in a plurality of links of the system is pulse code modulation B, and in which the repetition frequency of instants at which the amplitude of intelligence is observed is different as between different ones of the said plurality of links, and in which the predetermined fixed .v minimum change of amplitude as between one instant and the next necessary to cause the said change to be recorded in the modulation is dif- 

